Calling GetType on a Nullable type causes a boxing operation to be performed when the type is implicitly converted to Object. Therefore GetType always returns a Type object that represents the underlying type, not the Nullable type.

The C# is operator also operates on a Nullable's underlying type. Therefore you cannot use is to determine whether a variable is a Nullable type. The following example shows that the is operator treats a Nullable<int> variable as an int.

int? i = 5;
if (i is int) { ... } // true

You are correct in presuming that the C# compiler is optimizing nullable types. Here's a quote from Jon Skeet's C# in Depth which should answer your question:

It’s only with respect to boxing and unboxing that the CLR has
any special behavior regarding nullable types. In fact, the behavior was only changed shortly before the release of .NET 2.0, as the result of community requests.

An instance of Nullable is boxed to either a null reference (if it doesn’t have a value) or a boxed value of T (if it does). It never boxes to a “boxed nullable int”—there’s no such type.